The present invention relates to a temperature sensitive electromagnet (TSEM) for automatically inserting a retained control rod into a reactor core without any external control to effect emergency shutdown of the reactor when the temperature in the reactor rises in an extraordinary manner.
A fast breeder reactor is provided with a system for inserting the control rod into the reactor core when anything unusual occurs. As such a reactor shutdown system, there has been proposed a system for retaining and separating the the control rod by using an electromagnet, the magnetic force of which varies depending upon its ambient temperature. FIGS. 8 and 9 illustrate a concept of such a system.
An electromagnet 10 is provided with upper and lower iron cores 12, 14 capable of latching with and delatching from each other, and a coil 16 wound around the upper iron core 12. A temperature sensitive magnetic material (TSMM) 18 having a suitable Curie-point is incorporated in a part of the lower iron core 14. The upper iron core 12 is suspended from a control rod driving unit 20, and a control rod 22 is connected to the lower iron core 14. When the ambient temperature of the electromagnetic 10 exceeds the Curie-point of the TSMM 18, this TSMM 18 turns to non-magnetic, so that the magnetic resistance of the magnetic circuit becomes high. Consequently, the magnetic force decreases even when an electric current continues to be supplied to the coil 16. Accordingly, even if any external command is not given, the control rod 22 falls to be inserted into a reactor core 24.
This system is called a self-actuated reactor shutdown system, which is considered to be very much reliable in that it does not require processes, which are necessary for a prior art shutdown system, of monitoring conditions of the interior of the nuclear reactor and of issuing a command to insert the control rod into the reactor core when an abnormality is detected.
The principle of operation of the above-described system is simple, and the characteristics thereof have already been ascertained. However, application of this principle to an actual nuclear reactor has some problems. First, an electromagnet is disposed in an existing control rod guide tube, and, therefore, it is restricted severely with respect to the dimensions (90 mm in diameter in the case of, for example, a 1,000,000-kW class fast breeder reactor). It is necessary that a sufficiently large magnetic force in comparison with the weight of the control rod be secured under such dimensionally restricted conditions. Theoretically speaking, the electromagnet may have a magnetic force exceeding the weight of the control rod. However, if a margine of the magnetic force is insufficient, it is highly possible that the electromagnet erroneously actuates due to even a small-scale earthquake or the vibration of a fluid occurring during a normal operation of the reactor. This is a great disadvantage in operation of a plant.
On the other hand, in order to delatch the control rod, it is necessary that the magnetic force decreases sufficiently with respect to the weight of the control rod when the ambient temperature of the electromagnet is above the Curie-point of the TSMM. Unlike the electricity, the magnetism has a certain level of magnetization even in a space. Accordingly, even when the TSMM has become non-magnetic, a certain level of the magnetic force basically remains therein. In order to minimize the residual magnetic force, the volume of the TSMM should be increased. However, if the TSMM is enlarged excessively, the magnetic resistance in the magnetic circuit increases, and the level of the magnetic force at a normal operation temperature of the reactor decreases since the saturation magnetization of the TSMM is generally lower than that of the iron core material (iron). Especially, the electromagnet in a nuclear reactor shutdown system is required to generate large magnetic force in a limited space. Therefore, at a normal operation temperature of the reactor, the electromagnet is used in a state that the magnetic flux density thereof is close to a saturation magnetic flux density of iron. Consequently, the magnetic resistance of the TSMM, that is, the keeping the area of the interface between the iron and TSMM becomes an important problem. Since the magnetic flux passes through the TSMM at a normal operation temperature of the reactor, the magnetic resistance thereof depends upon the cross-sectional area and length of the TSMM just like the electric resistance thereof. However, since the TSMM becomes non-magnetic at temperatures higher than its Curie-point, the magnetic flux passes through the space without respect to the TSMM. Accordingly, the magnetic resistance of the iron core as a whole above the Curie-point is determined depending upon the shape of the iron core except the TSMM. Therefore, in order to cause the TSMM to exhibit excellent magnetic characteristics, it is necessary that the shape and the location of the TSMM must be carefully designed.
FIG. 10 shows an example (design example 1) of a lower iron core of an electromagnet. A TSMM 18a is disposed in the lower corner of the outer circumferential portion of the lower iron core, and slits 26a are provided for the purpose of improving the temperature response. In this structure, the area of the interface F.sub.1 of an iron core material 28a and the TSMM 18a in the outer circumferential side is secured by extending the interface F.sub.1 in the inward direction (to form an extension f of the interface). As a result, at a normal operation temperature, holding force the value of which is about three times as high as that of the weight of a control rod is obtained. When the TSMM has become non-magnetic at an ambient temperature above the Curie-point of the TSMM, the value of residual magnetic force becomes about 1/6 of that of the weight of the control rod, since the interface F.sub.1 is perpendicular to the central iron core to cause the TSMM and the central iron core to be apart from each other effectively. Consequently, sufficiently good magnetic force characteristic for the nuclear reactor shutdown system is obtained.
In addition to the excellent magnetic force characteristics, the electromagnet is required to have excellent temperature response such that the magnetic force rapidly lowers to delatch the control rod quickly when the ambient temperature exceeds the Curie-point. It is demanded in a fast breeder reactor that the self-actuated shutdown system actuates within delay time of about 2 or 3 seconds under the severest condition. In the above-mentioned design example 1, the temperature response is not so good. The reason resides in that, since a sufficiently large area of the interface between the iron core material and the TSMM has to be secured, deep slits cannot be formed, so that the inner portion without slits which has poor temperature response remains in the TSMM. In general, the temperature of fins 27a sandwiched by the slits 26a responds rapidly (temperature response is excellent) with respect to the change of the ambient temperature, but the temperature response of the inner TSMM portion without slits is very poor. The result of a magnetic field analysis shows that, assuming that the temperature of only fins 27a that have excellent temperature response exceeds the Curie-point, the level of the magnetic force of the electromagnet does not become lower than the weight of the control rod, which means that the temperature of some part of the inner TSMM portion without slits has to exceed the Curie-point to get sufficient reduction of the magnetic force. Further, the result of a thermal analysis shows that it takes not less than 6 seconds to reduce the magnetic force to the level required for the actuation of the electromagnet, even on the assumption that coolant flows ideally into the slits 26a.
Generally speaking, in order to improve the temperature response, the TSMM as a whole may be composed of a fin structure. However, such a structure means a fin structure wherein the area through which magnetic flux passes decreases to cause a shortage of magnetic force. Especially, since the saturation magnetization of the TSMM is lower than that of iron, the influence of such a fin structure appears distinctly. A shortage of the area of the interface between the iron core material and the TSMM causes the largest structural problem. A design example 2 shown in FIG. 11 has been devised so as to solve this problem. A TSMM 18b is sandwiched between inner and outer long iron core materials 28b, and the area of the interface F.sub.2 is thereby secured. In this structure wherein slits 26b and fins 27b are formed in the whole of the TSMM, the excellent temperature response can be expected, but there is a drawback that the magnetic force remaining at a temperature above the Curie-point becomes large. The results of measurement of the holding force in the design example 2 and the design example 1 are shown in Table 1.
TABLE 1 ______________________________________ Magnetic force Residual magnetic Electromagnet during a normal force at a temperature structure operation above the Curie-point ______________________________________ Design 217 Kg 10 Kg example 1 Design 130 Kg 30 Kg example 2 ______________________________________
As is apparent from Table 1, the ratio of the residual magnetic force to the holding force of the design example 2 becomes markedly higher than that in the design example 1. Incidentally, in order to obtain holding force the level of which is equal to that of the holding force in the design example 1, it is necessary in the design example 2 to supply a larger electric current to the coil. Consequently, the residual magnetic force becomes 60 kg which is equal to the weight of the control rod. Therefore, even when the ambient temperature exceeds the Curie-point, the delatching of the control rod becomes difficult. The reason resides in that, since the surface area between inner and outer iron core members is large, magnetic flux flowing through the surface between these iron core members becomes large even when the TSMM becomes non-magnetic at a temperature above the Curie-point. The retention and separation of the control rod is theoretically possible by the magnetic force in the design example 2 but the margin of the magnetic force in the design example 2 is small as compared with that in the design example 1. Therefore, the possibility of occurrence of spurious shutdown in normal operation of the reactor is large. In the design example 2, the magnetic force characteristics is sacrificed to improve the temperature response.
In order to prevent spurious shutdown (fall of the control rod) due to vibration during normal operation, a unique vibration absorbing mechanism has been developed. It has been ascertained that magnetic force of about 130 Kg which is two times as large as the weight of the control rod is sufficient to prevent spurious shutdown even if severer earthquake of S2-class occurs. However, in view of the condition in an actual nuclear reactor plant, it is preferable that, in the initial condition, the electromagnet has magnetic force of a level which is, with additional margin, at least about 160 Kg, i.e. at least 2.5 times as high as that of the weight of the control rod.
Regarding the temperature response, the following conditions are assumed as the severest requirements in a large-scale fast breeder reactor in which introduction of the reactor shutdown system has strongly been demanded. Namely, when the temperature of the coolant around the electromagnet increases 30.degree. C./sec, the delay time in actuation of the electromagnet should be less than 3 seconds. The rate of increase of temperature referred to above is determined on the assumption that an accident of coolant flow rate shortage and a failure of reactor shutdown by a conventional shutdown system occur concurrently, which represents the most rapid temperature increase in accidents. The delay time in actuation means the time between an instant at which the temperature of the coolant around the electromagnet reaches a predetermined operating temperature (operating temperature in a case where the temperature of the coolant increases at a slow rate) and an instant at which the electromagnet actuates in practice. The temperature response of the electromagnet is evaluated on the basis of this delay time in actuation.
As described above, in the electromagnets based on the design examples 1 or 2, either the holding force or the temperature response is sacrificed, so that the severest conditions for an electromagnet in a large-scale fast breeder reactor which demands both of these characteristic could not be satisfied.